Method of making selenium coated elements



Patented May 1 1951 MErHon F MAKlN ummon cousin ELEMENTS.

can E. Peters, University City,

Kirkwood, Mon assi'gnors to and David Rau, Vickers, :lnc'orpo rated, a cor orati n or Michigan No Drawins- Ap tion July 19, I947,

' and causing that crystalline layer to adhere to. a

supporting element. A

In making. a device which has selenium as a constituent part thereof, it. is customary to select a suitable supporting element and forma layer of selenium on thatelement. Considerable difficulty has been experienced in obtaining .a good physis cal andelectrical bond between the selenium and Serial No. 762,192

the supporting element, because selenium does not. readily adhere to most metals. However, it is quite essential that this difiiculty be overcome. and that a good physical and electrical bond be. attained between the selenium and the support.- ing element; particularly where the selenium,- coated supporting element; is to he used inmalgm rectifiers, photo-cells, or other electrical equipment. Various methods and apparatus have been employed in attempts to secure good Physi cal and electrical bonds between the. selenium and the supporting elements; and one of those meth ods included an initial treatment of the surfaces of the supporting elements, as. by sandblas in electroplating or spra coatin Such. reatments have been found to be of assistance; but in addition, it has b en found necessary to exe t pressure onth sel nium to press. it into fir n and. ad.- he in contact with the treated surf ces of he supportin e ements... Moreover t is usuall necessary to heat the sele um at. the time itis pressed i to close contact w th the. surfaces o the I up ortin elements; a d whil such heat-ins fes ters the adherence of the selenium to the supporting elements it also fosters the adherence of the selenium to the pressure members. Any such adherence is objectionable because it necessitates an additional step of separating the seleniumcoated plates, from the pressure members, it cessita'tes frequent cleaning of those members, and it may cause imperfections in the selenium surfaces.

In one commonly used method of applying selenium to a supporting element, heatis applied to the supporting element until the temperature thereof is above the melting point of selenium; and thereafter a stick of selenium o'r'a quantity 2 of powdered s'eleniumis brought into contact with the heated supporting element and permitted to melt. The molten selenium flows onto the surface of the supporting element and forms a coating, but that coating will not have the required uniformity of thickness or'adherences to the sup porting element. It has, therefore, been found necessary to apply pressure to the seleniumcoat ings, formed in this way, eitherin the presence of residual heat or in'the presence of later-applied heat to change the selenium to a smooth, uniform-thickness layer that will be firmly bonded to the supporting element. In another method, selenium powder or pellets of se'len'ium are placed on a heated supporting element and permi'ttedto melt. The molten selenium will tend to flow into a smooth layer, but the resultant coating will not be as smooth as required; and heat and pressure will have'to be applied to that coating to make it smooth and of uniform thickness. In still an other method, powdered selenium is placed on a cool supporting element, and that element is then placed in a heated press to change the powdered selenium into a smooth, uniform layer adherent to the supportingelement. The practice of each of these methods necessitates the positioning of a stout, s'mo'otmsurfaced pressure member in contact with a heated selenium anuthat plate must press the selenium into adhering engages. ment with the supporting element and yet it must not permit the selenium to adhere to it or chemically react with it. If either of these 0011*. tingenci'es were to occur, an extra step would have to be introduced into the method to. sepae rate the selenium-coated supporting elements from the pressure members, and in addition the separating step might cause imperfections in the selenium surface. In recognition of the fact that when temperature and pressure conditions tend to cause selenium tov adhere to one metal surface those same conditions tend to cause the selenium to adhere to. or react with other metal surfaces, most methods of pressin the selenium into ads. hering relation with the supporting element use non-metallic pressure members of mica, glass, heat-resistant plastic compositions such as phenol-formaldehyde resins, or oxidized aluminum. Having nonsmetallic surfaces, these pressure members are able to press the selenium mtoad hering relation with the supporting element and stillbe non-reactive with, and freely separable from, the selenium at the conclusion of the press: ing and bonding operation. However, mica and glass are not completely satisfactory because they are quite fragile and must be handled with great 3 care when used to apply heavy pressures in the presence of heat. Moreover, large sheets of mica are very expensive; and where large seleniumcoated supporting elements are to be prepared, the mica must be obtained in the form of large sheets. The heat-resistant plastic compositions are not satisfactory because they tend to soften or deform in the presence of heat and pressure;

and thus they cannot assure the attainment of smooth, uniform-thickness layers of selenium. The oxidized aluminum member can be made to possess the required strength and toughness; but it too is unsatisfactory because the heated selenium tends to affect the oxide film, thus requiring frequent cleaning of the pressure member. and

the formation of a new oxide film or surface. In.

addition, the oxidized aluminum scratches rather readily, thus requirin frequent replacement. Moreover, the oxidized aluminum pressure members have been found to affect the electrical characteristics of the selenium coating to some extent. For these various reasons previously used pressure members, and the methods of using them in makingselenium-coated supporting elements, areunsatisfactory andare objectionable. The present invention obviates these objections by providing a stout metal pressure member that can be placed in contact with selenium and used to press that selenium into adhering relation with a supporting element without itself adhering to or reacting with the selenium. It is therefore an object of the present invention to provide a metal pressure member to which selenium does not readily adhere and with which selenium does not readily react.

The present invention does this by incorporating sizable percentages of chromium in the metal used in making the pressure members. The chromium materially reduces the likelihood that selenium will adhere to or react with the metal of the pressure member. Thus it is possible to provide a stout, metal, pressure member that can be used to make selenium adhere to a supporting element. It is therefore an object of the present invention to provide a, chromium-containing metal pressure member that canbe used in making selenium-coated supporting elements.

the manufacture of devices whi h have selenium as constituent parts thereof it is frequently desirable that the selenium be converted to the grey crystalline state. That conversion can be performed conveniently at the time the selenium coating of thesupporting element is being annealed, after its formation in the presence of heat and pressure. The conversion of the amorphous selenium to the crystalline state requires a certain amount of time; and it would be .helpful if that time could be reduced. The

present inventionprovidessuch a reduction in conversion time by using a pressure member,

during the forming and pressing step, which encourages the formation 'of crystal nuclei in the selenium coating. Those nuclei encourage the growth of the selenium crystals in the subsequent annealing step. It is, therefore, an object of the present invention to provide a pressure member that resists reaction or adherence with selenium and that encourages the formation of crystal nucleii.

Other and further objects and advantages of the present invention should become apparent from an examination of the accompanying description.

The accompanying description discloses two preferred methods of practicing the present in- 4 vention but it is to be understood that the ac companying description is for the purposes of illustration only and does not limit the invention and that the invention will be defined by the appended claims.

One preferred method of providing a supporting element with a thin, adherent, uniformthickness coating of selenium is disclosed in our patent application that is entitled Devices Which Have Selenium as Constituent Parts Thereof and was filed. May 5, 1947, and bears Serial No. 746,164, now abandoned. In that method the supporting element is heated until it reaches a temperature suificient to cause selenium to react with it and form a selenide, and thereafter selenium is brought into engagement with the supporting element and permitted to form a selenide on the surface of that element. The supporting element is then cooled, and when 0001 it is provided with a layer of powdered selenium; thereafter the supporting element is placed in a press where the powdered selenium is heated to temperatures of about one hundred and thirty degrees centigrade and pressed under pressure of about three hundred and fifty (350) pounds per square inch until it is changed to a uniform-thickness layer adherent to the supporting element. The selenide will facilitate the adherence of the powdered selenium to the supporting element, and the press will smooth the selenium into a, uniform-thickness coating. In this method the powdered selenium must be directly contacted by the pressure member of the press, and that member must force the selenium into adhering relation with the supporting element and yet must not itself adhere to or react with the selenium. That pressure member must also be strong and unyielding so it can make a smooth, uniform-thickness coating from the layer of discrete selenium powder particles. In addition, that pressure member should form a number of crystal nuclei on the surface of the selenium coating. Such a member can be made from a hard metal which has a chromium content of fifteen (15) percent or greater. Fifteen (15) percent is probably the lowest usable percentage of chromium, because in alloys having a lesser chromium content the other metal of the alloy, which is usually more reactive than chromium, is in sufficient concentration to react with the selenium. When a pressure member with fifteen (15) percent chromium or more has a high percentage of iron, that member can form crystal nuclei on the surface of the selenium as it presses the selenium into adhering relation with the supporting elements. Such a pressing member avoids the fragility of glass and mica, and it avoids the excessive cost of large mica sheets. Moreover, it avoids the softness and distortion of the heat-resistant plastic compositions, and it also avoids the frequent cleaning and replacement needed with the oxidized aluminum pressure members. Such a pressure member makes it possible to use sufficient heat and sufiicient pressure to attain good physical and electrical adherence between the selenium and the supporting elements; and it also, because of its resistance to adherence and to chemical reaction, provides a smooth, clean surface for the selenium. Moreover it reduces the time required to convert the amorphous selenium to the grey crystalline form.

It is not meant to infer that the selenium tends to adhere to pressure members with such force h t e fo c re i ed s S a t the seleniumf the rejection of the elements or by requiring an .2

additional p of p ying the coated su porting elements from the pressure member or by requiring frequent cleaning or replacement of the pressure member.

"Anothermethod of forminga smooth uniformthickness coating on a, supporting element utilizes a one-step application of selenium to the supporting element. This method is an even more severe test of the value of the pressure member of the present invention because the temperatures and pressures in that method have to be selected to enable the selenium to adhere directly to the surface of the supporting element. In that method the supporting element is heated to a temperature above the melting point of selenium, and thereafter selenium is brought into contact with the heated supporting element and is permitted to melt. Here again, to assure good ad herence between the selenium and the supporting element, a pressure member must be placed in contact with the selenium, in the presence of residual or later-added heat, and used to press the selenium against the supporting element. In this method the pressing of the selenium against the supporting element is done at pressures of about one hundred and fifty (150) pounds per square inch and at temperatures of about one hundred and fifty (150) degrees Centigrade. In this second method the problem of forming crystal nuclei is also quite important.

In the first method the selenide, provided by the first step of that method, constituted a surface to which the second layer of selenium could easily adhere; thus reducing the temperature required and also reducing the tendency of the selenium to adhere to the pressure member. No such surface is provided in the second method, and as a result, the task of making the selenium adhere directly to the supportin element without also adhering to the pressure member is more difiicult; because if the temperature and pressure can make the selenium adhere to the metal of the supporting element, they will also tend to make the selenium adhere to the metal of the pressure member. Where the second method of making seleniumcoated supporting elements is used, it is preferable, therefore, to employ a pressure member with an increased chromium content. One such pres- 7 sure member is a stainless steel plate with a chromium content of thirty (30) percent or more; and such a pressure member obviates adherence between the selenium coating and the pressure member, and it also fosters the formation of crystal nuclei. Thus it is possible to produce selenium-coated supporting elements emciently and with a minimum of cost and maintenance.

The metal of which the pressure members is made will preferably be half-hard or harder, and where that is done the pressure members will be resistant to scratching. In addition, the pressure members will be quite resistant to deformation.

In some instances, where the amount of time required to convert the amorphous selenium to the grey crystalline form can be increased without interfering with production schedules, it is possible to use a chromium-plated pressure mem ber. :Such a member will resist reaction or adherence with the selenium on the supporting elements, but it will not be capable of providing the extensive growth of crystal nuclei obtainable through the use of chromium-containing ferrous plates. Such growth of nuclei not only reduces the time required to convert amorphous selenium into the grey crystalline state, but it has in some instances reduced the forwardresistance of the selenium coating without reducing the inverse resistance of that coating.

fhe chromium-containing ferrous-plates provided by the present invention have beenin daily use in commercial production for many months, any they show no signs of needing resurfacing :or replacement. Those plates eliminate the objections found in connection with glass, mica, heatresistant plastic compositions, and aluminum, and in addition they foster the growth of crystal nucleii. There is no tendency for the chromiumcontaining surfaces of the pressure members to adhere to or react with the selenium, and the plates will remain clean for very long periods of time.

The ability of the chromium-containing ferrous pressure members to foster the growth of crystal nucleii can be further enhanced by providing the surface of those members with a metallographic etch. One etching solution that has been found to be useful includes ten (10) grams of ferric chloride, thirty (30) cubic centimeters of hydrochloric acid, and ninety cubic centimeters of water; and that solution provides a desirable etch after a reaction of thirty (30) seconds.

Where there is any expectation that the selenium, used in forming the coating on the supporting element, may flow onto the platten or other portion of the press, that platten or other portion of the press should also be made of a chromium-containing metal or should have a chromium plated surface. In such instances, the excess selenium can easily be removed from the surfaces of the press.

Whereas two preferred methods of practicing the invention have been disclosed in the accompanying description it should be obvious to those skilled in the art that various changes may be made in the method of practicing the invention Without affecting the scope thereof.

What we claim is:

l. The method of making a selenium-coated supportin element with a smooth, adherent, uniform thickness nuclei-containing coating of selenium thereon which comprises depositing a thin coating of selenium onto a supporting element to which selenium will readily adhere, placing in contact with said selenium coating a pressure member which consists essentially of iron and which contains between fifteen percent (15%) and about thirty percent (30%) chromium and which has a metallographic etch on the surface thereof, forcing said pressure member against said selenium coating under a pressure of from one hundred and fifty (150) to three hundred and fifty (350) pounds per square inch and at a temperature of from about one hundred and fifty (150) to about one hundred and thirty degrees centigrade respectively until said selenium adheres to said supporting element and crystal nuclei are formed on the surface of said,

7. selenium coating, and separating said pressure member from said selenium coating.

2. The method of making a selenium-coated supporting element with a smooth, adherent, uniform thickness nuclei-containing coating of selenium thereon which comprises depositing a thin coating of selenium onto a supporting element to which selenium will readily adhere, placing in contact with said selenium coating a pressure member which consists essentially of iron and which contains between fifteen percent (15%) and about thirty percent (30%) chromium, forcing said pressure member against said selenium coating under a pressure of from one hundred and fifty (150) to three hundred and fifty (350) pounds per square inch and at a temperature of from about one hundred and fifty (150) to about one hundred and thirty (130) degrees centigrade respectively until said selenium adheres to said supporting element and crystal nuclei are formed on the surface of said selenium coating, and separating said pressure member from said selenium coating.

CARL E. PETERS. DAVID W. RAU.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,066,611 Christy Jan. 5, 1937 2,339,613 Becker Jan. 18, 1944 2,342,278 Herrmann Feb. 22, 1944 2,424,995 Miller Aug. 5, 1947 

1. THE METHOD OF MAKING A SELENIUM-COATED SUPPORTING ELEMENT WITH A SMOOTH, ADHERENT, UNIFORM THICKNESS NUCLEI-CONTAINING COATING OF SELENIUM THEREON WHICH COMPRISES DEPOSITING A THIN COATING OF SELENIUM ONTO A SUPPORTING ELEMENT TO WHICH SELENIUM WILL READILY ADHERE, PLACING IN CONTACT WITH SAID SELENIUM COATING A PRESSURE MEMBER WHICH CONSISTS ESSENTIALLY OF IRON AND WHICH CONTAINS BETWEEN FIFTEEN PERCENT (15%) AND ABOUT THIRTY PERCENT (30%) CHROMIUM AND WHICH HAS A METALLOGRAPHIC ETCH ON THE SURFACE THEREOF, FORCING SAID PRESSURE MEMBER AGAINST SAID SELENIUM COATING UNDER A PRESSURE OF FROM ONE HUNDRED AND FIFTY (150) TO THREE HUNDRED AND FIFTY (350) POUNDS PER SQUARE INCH AND AT A TEMPERATURE OF FROM ABOUT ONE HUNDRED AND FIFTY (150) TO ABOUT ONE HUNDRED AND THIRTY (130) DEGREES CENTIGRADE RESPECTIVELY UNTIL SAID SELENIUM ADHERES TO SAID SUPPORTING ELEMENT AND CRYSTAL NUCLEI ARE FORMED ON THE SURFACE OF SAID SELENIUM COATING, AND SEPARATING SAID PRESSURE MEMBER FROM SAID SELENIUM COATING. 